The invention relates to a power supply circuit having input terminals for receiving an input voltage and output terminals for supplying an output voltage, which circuit comprises, coupled parallel to the input terminals of the power supply circuit, a series arrangement of a primary winding of a transformer and a switch, a secondary winding of the transformer coupled to the output terminals of the power supply circuit and a control circuit having an input which is coupled to the output terminals of the power supply circuit and an output for applying a control signal to the switch, said control circuit comprising an oscillator circuit for generating a pulse width-modulated control signal having a predetermined frequency.
The invention also relates to a control circuit for use in such a power supply circuit.
A power supply circuit of this type is known from German Patent Specification DE-C-3444035.
To ensure that an output voltage does not change independently of load variations, pulse width modulation (PWM), also referred to as duty cycle control, is generally used. A drawback of pulse width modulation (duty cycle control) is that the width of the pulse has a minimum. If the load decreases, the width of the pulse is reduced so as to keep the output voltage equal. Since the pulse is applied to a control electrode of, for example a transistor operating as a switch, it will take some time at the moment of the start of the pulse before the switch is rendered conducting. Moreover, at the end of the pulse it will take some time before the switch becomes non-conducting. At a given pulse width the switch will be conducting for a given minimum period of time and a further reduction of the pulse width does not have any influence on the conductivity time of the switch, until the width of the pulse becomes so narrow that the switch is not rendered conducting at all anymore.
Apparatuses such as television receivers, etc. in particular are in a standby mode for the greater part of the time. During this time only a small part of the apparatus is power supplied, inter alia, far example the reception section of the remote control unit. A general object is to ensure that the apparatuses in the standby mode consume a minimum quantity of energy. Due to this object, the width of the pulse in the standby mode becomes increasingly narrower when using pulse width modulation, so that the above-mentioned problems arise, or even get worse. Also in applications in which a (mains-supplied) apparatus requires a small quantity of power during a given part of the time (which need not be the above-mentioned standby mode) the above-mentioned problems occur. This problem is solved in the above-mentioned German Patent Specification by switching over from pulse width modulation to pulse packet control after the load has been detected to be below a given value. The pulse packet control implies that a plurality of pulses is successively applied at equal intervals and at a fixed width (wider than the minimum width) to the switch, whereafter no pulses are generated for a given period of time. The number of pulses is dependent on the load. A drawback of the known control is that the voltage generated by means of the switch and a conventional (primary winding of a) transformer (or wire-wound coil) should be extensively filtered to make the voltage suitable to be applied to a load, which filtering is necessary because the generated voltage has a large variation (ripple).
A further drawback of this known control is that audible transformer noise occurs. This is a vibration of the core (caused, inter alia by the air gap) and of the wires of the windings of the transformer, which vibration occurs in the transformer due to a low-frequency varying magnetic field.